Arvid's first CNC machine!

[arvidb]

Originally posted by balsaman
Your speed will be limited to what your pc can pump out. I think 35 khz is max for mach 2. Turbocnc maxes out around 25khz. 250-500 cpr encoders would be better. I think the G340 drive has a pulse devider for high count encoders.

These motors are 3-ph AC, and thus won't run with Gecko drivers. The drivers I can get that will run these motors lack a position servo loop (i.e. they have only the torque and speed servo loops). So I will have to build my own position PID controller, and could implement a count multiplication/division in this as needed.

This way I could set things up so that 1 step from the PC gives a movement of, say, 0.005 mm (=> 0.1 m/s @20 kHz), but still use the full encoder resolution for positioning accuracy.

BTW, a 500 counts/rev encoder with a 10 mm/turn screw, and 2.5:1 motor-to-screw reduction gives a speed of 0.16 m/s or 375 IPM @20 kHz! Or 755 IPM for a 250 counts/rev encoder! A bit fast perhaps?

// Arvid

[HuFlungDung]

Originally posted by arvidb
Hi everybody! I hope you all are having a nice christmas!

I need some advice about timing pulleys. My motors have 14 mm axles, the screws have 10 mm axles. I want about a 2.5:1 reduction. Also, I need a type with low backlash.

Do you guys drill/turn your own bores in the pulleys? If so, how to do that without a lathe? And will a setscrew be enough to keep the pulleys from skidding on the axles? The axles are all smooth, no flats.

I haven't been able to find anything that would fit my axle dimensions and reduction ratio .

// Arvid

Hi Arvid,

Have you checked with a supplier like Browning or Gates? They manufacture two piece timing pulleys with taperlock bushings which I think you can buy to match standard shaft sizes, both inch and metric. There is no backlash in a timing belt drive, unless your slides were extremely stiff for some reason. I have used timing belts on many servo drives and never had any backlash attributable to the belt and pulleys.

[arvidb]

Thanks for the tip! Those taperlock bushings look really nice, just what I was looking for!

I haven't found anything that fits my 10 mm screw journals yet, but now at least I know what to look for.

// Arvid

[arvidb]

Hi everybody,

I thought I would report on the situation:

I'm still looking around for a timing transmission. I'm having a thorough email exchange with a local manufacturer of timing belts and pulleys, and they gave me some valuable suggestions.
First, they recommend the AT tooth profile with polyurethane belts to get minimum backlash. If I understand correctly, HTD is worst in this respect, followed by T, GT, and AT, in that order.
Second, to get good grip on smooth journals, one can use either taperlock bushings or another kind of bushing that sits between the bore of the pulley and the journal. For an example, see "Heavy Duty Keyless Bushings" on the McMaster-Carr website. Taperlock would be nice, but I don't think there exists taperlock pulleys for my small journal dimensions.

The only problem so far is that a complete, new timing belt transmission of this kind is 2-3 times what I can afford. Maybe I can find something at ebay.

Also, I have finally found some time to do CAD drawings of my ball screws and some other components. The timing pulleys are very simply drawn, since I don't know exactly what kind they will be. They are the same size as AT5 pulleys with 22 and 60 teeth, resp. Here is a more detailed look of the "bridge bar" (without the Z slide and guides because I don't have them yet!):

It's a bit incomplete; for example the only ball bearing that's got a complete suspension is the far Y axis one.

[ger21]

From what I understand, GT2 belts have the least amount of backlash.

Gerry

[Hobbiest]

Hey Arvid and Mr. ED,
www.Igus.com, the company is based in Stuutgard Germany (did I spell that right?) Might have distribution from that home location also!
Arvid, What CAD program do you use, and how user friendly would it be to a moron? I have tried a couple, but they are very difficult to get the hang of, and make really cheesy looking drawings.
Igus' stuff is REALLY nice!!

[Mike F]

Hobbiest

Take a look at ProdeskTop Express from PTC.com for a CAD package, it's free!!!! It is a cut down version of ProEngineer and I wouldn't mind betting it will do all you want it to.

This software has been made available to all UK schools, complete with support, and is an excellent package for nothing, well worth a look.

Mike

[Mike F]

Sorry! Meant to attach a drawing done with ProdeskTop Express.

[arvidb]

Originally posted by Mike F
Hobbiest

Take a look at ProdeskTop Express from PTC.com for a CAD package, it's free!!!! It is a cut down version of ProEngineer and I wouldn't mind betting it will do all you want it to.

Mike

Unfortunately, as of the new year, PTC has discontinued the free version of Pro/DESKTOP Express. You can no longer get the activation code needed to run the program. And no use asking anyone else for it either, since the code is unique for each installation :-(

I'm using Pro/DESKTOP Express, but if (when!) I ever need to re-install my computer, well then that's it!

// Arvid

[arvidb]

Originally posted by ger21
From what I understand, GT2 belts have the least amount of backlash.

Gerry

Actually, I haven't found any real information on this! Do you have any comparison data?

This is from the "Gates Poly Chain GT2 Drive Design Manual":
"PowerGrip®HTD® Drives have improved torque carrying capability and resist ratcheting, but have a significant amount of backlash. Polychain GT2 Drives have considerably improved torque carrying capability, and backlash characteristics in between that of PowerGrip HTD and PowerGrip Timing Drives."

This seems to be a comparison between the Polychain GT2 and Powergrip GT/GT2 products. It's a bit unclear, but indicates that the "registration" (or backlash) is similar between the GT and the GT2 (Powergrip).

From Stock Drive Products (SDP) "THE WORLD OF TIMING BELTS":
"Trapezoidal Timing Belt Drives are known for having relatively little backlash. PowerGrip HTD Drives have improved torque carrying capability and resist ratcheting, but have a significant amount of backlash. PowerGrip GT Drives have even further improved torque carrying capability, and have as little or less backlash than trapezoidal timing belt drives."

The T belt profile is a trapezoidal timing belt. The AT is an improved version designed especially for low backlash (the AT is curvilinear, but close to trapezoidal, I think). So I draw the conclusion that AT is the best there is in this regard. The GT2 would be better in all other areas, though, I guess

If anyone's got better info regarding this, I'd very much like to share it!

// Arvid

[Hobbiest]

Mike F,
Thanks for the info...I'll still check into it. What is that drawing? Some kind of one way check valve?

[HuFlungDung]

Here is another belt to check on: Eagle PD synchronous belts by Goodyear. This is a "herringbone" style of belt which is supposed to be much quieter, perhaps 15 dba. The pulleys do not need flanges because of this belt tooth pattern. For the "white" belt series (second to narrowest belt), the backlash is .8 degrees on 22 tooth sprocket.

[Mike F]

Spot on Hobbiest. It's a non-return valve I made for a vacuum bagging, prepreg oven here in my workshop. I build competition model aircraft (gliders), machine the patterns with CNC then make moulds and production items using prepreg composites in the oven. Keeps me off the streets.

I intend to make a much larger, approx 2000mm CNC for making larger wings. Looking on ebay at this moment for bits.

Mike

[ger21]

IN my SDP/SI Handbook of Timing Belts and Pulleys, "On drives using a low installation tension, small pulleys, and light loads, the backlash of the Powergrip GT2 system will be slightly better than the trapezoidal timing belt system. Hpwever, with increased tension and/ or loads and/or pulley sizes, the performance of the Powergrip GT2 system becomes significantly better than the trapezoidal timing belt system. "

This is the paragraph right before the one you quoted. The only problem with this info is that they have NO info on any other types of belt. Your guess is as good as mine.

Gerry

[trent2530]

Say Balsaman, what happened to your roller skate bearings?

Trent

PS: I just bought some drawer slides from Home Depot mayself. Experimenting.

[Hobbiest]

Arvid...use a file to cut flats on you shafts. Don't use a grinder...to much heat.

[arvidb]

Hobbiest,

thanks for the tip. I'm now looking into using tapered bushings. These clamp down very well on plain shafts, but may be a bit too expensive. If I cannot get these for a reasonable price, I may go with your idea. I guess you mean to use a set screw that "sets" on the flat?

Arvid

[Hobbiest]

Call Igus and talk to a sales rep. Have them send you a sample of some of their products. They sent me a sample of their bushings for round rod, and I tried it out on zinc plated rod from the hardware store. Also tried it on polished Thomson rod. The zinc rod worked a lot better! They say that their bushings work better with rougher shaft, and this means that you don't have to spend an arm on shaft, and a leg on bearings! I just got my set of bushings, 8 3/4" open, and 4 1/2" closed, and with shipping it was only like $30! Good deal so far.

[arvidb]

Hobbiest,

I think we might be talking about different things here. I'm talking about bushings for fastening timing pulleys to screw journals, while I think you are talking about linear guides?

I actually did contact Igus a few months ago, asked them a few questions and got their catalogue. I think their stuff looks very good for things like balsa cutting - very tolerant of dust and misalignment, cheap, good precision under light loads, and not least, looks good. It would be my #1 choice for a light machine.

Unfortunately they explicitly states that you should not use DryLin:
* when you need a high precision, preloaded linear system
* for heavy load applications

so I went for used THK style guides instead. With a lot of patience you can find those used for the same amount of money you would pay for new DryLin guides.

Arvid

[balsaman]

Originally posted by trent2530
Say Balsaman, what happened to your roller skate bearings?

What rollerskate bearings? I have never used any on a cnc.

E